The objectives of this project are to understand the process of muscle development and to establish fine structure-function relationships for myofilament proteins through the analysis of muscle-defective mutants. We are now focusing on the analysis of gene expression in single muscle fibers of the rabbit. Use of modified silver staining technique enables us to analyze the isozymic forms of myosin light chains (mics), components of the troponin complex (TnI, and TnT and TnC), tropomyosin (Tm), alpha-actinin, C-protein and titins from short segments of individual muscle fibers. We focused on two problems: 1) testing the "quantal" or exclusive model of fast and slow isozyme expression (rev: Duhoot and Perry, Nature, 278, 714 (1979): and 2) identifying new isozymic species of proteins. We have identified at least 2 species of TnT, 3 species of C-protein and 2 species of alpha-actinin not previously reported. The correlation of isozyme expression with fiber histochemistry has enabled us to identify 2 types of slow muscle fibers and 3 three types of fast muscle fibers. We conclude that muscle exists primarily in a limited number of states. So far, we can identify 5 such states. The correlation between the expression of the various isozymic forms leads us to propose that muscle genes are multigene families of which there are at least 5, and that each muscle choses one of those 5 sets of genes for expression.